This colorful specimen contains blue-green botryoidal rosasite that is encrusted in a layer of calcite. It was collected from the Ojuela Mine in Durango, Mexico
Rosasite is a secondary mineral that forms in oxidation zones of copper-zinc deposits, generally forming a fibrous botryoidal sphere that can appear vitreous and silky. While typically featuring a blue-green color, colorless specimens are not unheard of. The chemical formula of rosasite is CuZnCO3(OH)2.
Calcite, CaCO3, is a carbonate mineral and the most stable polymorph of calcium carbonate. The other polymorphs are the minerals aragonite and vaterite. Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra, tabular forms, and prisms. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form.